What is hyperconjugation and how do you know if there is hyperconjugation in a compound? Which orbitals are involved in hyperconjugation?
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3 Answers
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In the ethyl cation, one can write the following resonance structures:
The structure on the right can be actually written 3 times since there are 3 hydrogens available. Note also that the "ethylenic-like" structure on the right involves a $\mathrm{sp^3-p}$ bond (the $\ce{C-H}$ bond is $\mathrm{sp^3}$ hybridized and the cationic carbon has a $\mathrm{p}$ orbital), so it is not a true pi bond ($\mathrm{p-p}$) like that found in ethylene. The resonance structure on the right is meant to show the "hyperconjugation" effect of the $\ce{C-H}$ bond ($\mathrm{sp^3-s}$) with the $\mathrm{p}$ orbital located on the adjacent cationic carbon. The drawing is a shorthand way of saying that when the $\ce{C-H}$ bond is rotated into the same plane as the $\mathrm{p}$ orbital on the cationic carbon, there can be a stabilizing overlap - and it is called "hyperconjugation." In the 2-propyl cation, there are 6 of these hyperconjugative resonance structures (there are 6 $\ce{C-H}$ bonds on the 2 methyl groups adjacent to the cationic carbon and in the t-butyl cation there are 9 such resonance structures. This can be used to explain why tertiary carbonium ions are more stable than secondary carbonium ions, which in turn are more stable than primary carbonium ions.
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$\begingroup$ Your answer is really very insightful. I would just like to ask if hyperconjugation only exists only if there is a positive formal charge on one of the atoms in the molecule. $\endgroup$ Commented Jul 24, 2017 at 7:00
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$\begingroup$ @ron can more than one $\ce{C-H}$ bond overlap with one vacant $\mathrm{p}$ orbital? for example, in the t-butyl cation, can $\ce{C-H}$ bonds from the three different methyl groups overlap with the vacant $\mathrm{p}$? $\endgroup$– MuhammadCommented Jan 27, 2019 at 3:13
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Hyperconjugation is the interaction between adjacent orbital (empty $\mathrm{p}$ for cation, $\pi$ for alkene) with $\sigma$ bond, while common conjugation is between adjacent orbital and $\pi$ bond.
As one example, one reason why more alkyl substituted alkenes are more stable than less substituted ones is because more alkyl groups provide more hyperconjugation stabilization of the alkene. Why more alkyl substituted cations are more stable is also because the hyperconjugation stabilization from more adjacent alkyl group.
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Hyper-conjugation is in general a stabilizing interaction which involves the delocalization of σ-electrons belonging to the $\ce{C-H}$ bond of an alkyl group attached to an atom with an unshared p-orbital. The σ-electrons of a $\ce{C-H}$ bond of an alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p-orbital.
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$\begingroup$ @ParthChauhan Chemical markup can be applied easier and more correct using the mhchem extension. If you want to know more, please have a look here and here. $\endgroup$– Martin - マーチン ♦Commented Aug 6, 2018 at 14:31